1. Field of the Invention
The present invention relates to patch forming for measuring a density, a color value, or the like output from an image forming apparatus.
2. Description of the Related Art
Recent image forming apparatuses for forming color images have been operating as follows. First, a plurality of electrophotographic process units each having a charging device and a development device around a photosensitive member are provided side by side in an apparatus. On the surface of the photosensitive member in each process unit, a latent image is formed by forming an optical image on the surface using an exposure device emitting a laser beam, light-emitting diode (LED) light, or the like. The latent images are visualized using toner by the individual development devices, and sequentially transferred on an intermediate belt. The images are fixed on a medium and a color image is formed.
Meanwhile, further increase of the quality of the output images formed by the image forming apparatuses employing an electrophotographic method such as color printers and color copying machines have been demanded. Especially, the gradation of the density and the stability of the density influences human decision on the quality of an image.
However, if conditions of the units in the color image forming apparatuses vary according to environmental changes or long-term use of the apparatuses, the densities of the acquired images vary. Especially, in the case of color electrophotographic image forming apparatuses, a slight environmental variation causes a variation in the density, and as a result, the color balance may be lost. Accordingly, means for always keeping a constant density-gradation characteristic is to be provided. For the purpose, with respect to each color toner for printing, for example, process conditions such as several kinds of exposure amounts corresponding to absolute humidity, and development biases, a gradation correction unit such as a look-up table (LUT), or the like are provided. Based on absolute humidity measured by a temperature and humidity sensor, a corresponding process condition and an optimum value for the gradation correction are selected.
Further, in order to obtain a constant density-gradation characteristic regardless of the variations in each device, toner patches for density detection are formed using the toner of each color on the intermediate transfer member, the drums, or the like. The densities of the unfixed toner patches are detected by a sensor detecting an unfixed toner density (hereinafter, referred to as density sensor). According to the detection results, feedback is given to the process conditions such as an exposure amount and a development bias and thereby the density control is performed to acquire a stable image. Hereinafter, the density control is referred to as calibration.
However, in the density control using the density sensor, only the density of the patch formed on the intermediate transfer member or the drum is detected, and a change of the color balance caused by the transfer or fixation onto the medium performed after the density detection is not controlled. The color balance also changes due to the transfer efficiency in the transfer of the toner image onto the medium or the application of heat or pressure by the fixation processing. It is not possible to deal with the change by the calibration processing using the density sensor. Further, in the electrophotographic method, even if the density correction is performed on each color of cyan (C), magenta (M), yellow (Y), and black (K) for stabilization, the mixed color made by combining the colors tends to show deviation.
To solve the problem, some apparatuses form gradation patches of each color of cyan (C), magenta (M), yellow (Y), and black (K) or a patch of the mixed color of C, M, Y. and K on the medium. The apparatuses are provided with a sensor for detecting a density, a chromaticity, or a spectral value of the patch image on the medium after fixation of the patches. Such image forming apparatuses can form a multidimensional LUT using the read value, and perform more accurate color mixture calibration than a monochromatic one-dimensional LUT. The sensor for detecting the density, the chromaticity, or the spectral value of the fixed patch image includes color sensors and spectral sensors. A spectral sensor is described herein as an example.
The sensor (hereinafter, after-fixation sensor) for detecting a spectral value of the patch fixed on the medium is disposed on a sheet conveyance path between the fixation position and the discharge position of the sheet in the image forming apparatus. The sensor reads an output chart for gradation correction. The position of the sensor in the main scanning operation is fixed, and consequently, the patches can be added only in the sub scanning direction (the conveyance direction of paper).
Depending on the density of the patch, to read a spectral value of the patch, a length (accumulation time) longer than or equal to a certain length is to be given to the patch. For example, if a length about 17.5 mm is necessary for one patch, in a case where patch images are printed in the longitudinal direction of a medium of the A3 size, only up to 23 patches can be arranged. If the image forming apparatus supports media up to the A4 size, only up to 16 patches can be arranged in the longitudinal direction.
To perform color correction of a mixed color using the spectral sensor, patches of about 80 to 300 are to be provided. Accordingly, if only one spectral sensor is provided, to print 300 patches, even if the patches are printed in the A3 longitudinal direction, 14 sheets of the medium is to be supplied. If more spectral sensors are provided in the main scanning direction, the number of patches printable on one sheet of the medium can be increased. However, generally, the spectral sensor for measuring the spectral values of the patches is expensive, and consequently, the cost increases.
To solve the problem, the after-fixation sensor may be provided on the conveyance path in two-sided printing, and patches may be formed on both sides to decrease the number of sheets of the medium to be consumed for the calibration. However, in a case where the patches are formed at positions where the patches overlap on the front surface and back surface of the medium, the patches on the back surface may affect the density, the chromaticity of the patches, or the accuracy of the spectral value detection on the measurement side. To solve the problem, Japanese Patent Application Laid-Open No. 2008-008967 provides a technique for eliminating the show-through effects of the overlapping patches on the front surface and the back surface by forming the patches such that the patches do not overlap with each other on the front surface and the back surface, and displacing the position of the after-fixation sensor or the position of the medium being conveyed.
However, in the known technique, the mechanism of the device for displacing the after-fixation sensor or the medium in reading the patches formed on the front surface and the back surface of the medium requires cost increase. Moreover, the control of the movement of the mechanism becomes complicated. Moreover, to increase the reliability and the accuracy of the displacement of the devices, additional costs are required.